In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-c alled primary recovery methods which utilize only the natural forces present in the reservoir. A variety of supplemental techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected-water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil received by waterflooding. It has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant waterflooding or as low tension waterflooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Problems with stability and effectiveness have arisen when surfactants and thickeners have been used in relatively harsh environments such as those characterized by temperatures in the range of about 15.degree. C. to about 120.degree. C. and above, high pressures (e.g., up to about 4000 psi), high concentrations of divalent metal ions such as calcium, magnesium, etc. (e.g., up to 3000 ppm or more and in some instances as high as 10,000 or 20,000 ppm), and high salinity (e.g., total dissolved salts (TDS) levels of up to about 200,000 ppm).
The use of certain anionic surfactants for waterflooding have been suggested. For example, a paper by W. R. Foster Entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp. 205-210, describes a technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion. One problem encountered in waterflooding with anionic surfactants such as petroleum sulfonates is the lack of stability of these surfactants in so-called "hard water" environments. These surfactants tend to precipitate from solution in the presence of relatively low concentrations of divalent metal ions such as calcium and magnesium ions. For example, divalent metal ion concentrations of about 50 to about 100 ppm and above often tend to cause precipitation of the petroleum sulfonates.
Nonionic surfactants, such as polyethoxylated alkyl phenols, polyethoxylated aliphatic alcohols, carboxylic esters, carboxylic amides, and polyoxyethylene fatty acid amides, have a somewhat higher tolerance of polyvalent ions such as calcium or magnesium than do the petroleum sulfonates. However, while it is technically feasible to employ a nonionic surfactant solution to decrease the interfacial tension between the injected aqueous displacing medium and petroleum contained in some limestone formations, such use is generally not economically feasible for several reasons. Nonionic surfactants are not as effective on a per mole basis as are the more commonly used anionic surfactants and, additionally, the nonionic surfactants generally have a higher cost per unit weight than do the anionic surfactants. The polyethoxylated alkyl phenol nonionic surfactants usually exhibit a reverse solubility relationship with temperature and become insoluble at temperatures above their cloud points; making them ineffective in many oil formations. Nonionic surfactants that remain soluble at elevated temperatures are generally not effective in reducing interfacial tension. Other types of nonionic surfactants hydrolyze at temperatures above about 75.degree. C.
The use of certain combinations of anionic and nonionic surfactant to combat hard water formations has been suggested. For example, U.S. Pat. No 3,811,504 disclosed the use of three component mixture including an alkyl or alkylaryl sulfonate, an alkyl polyethoxy sulfate and a polyethoxylated alkyl phenol. U.S. Pat. No. 3,811,505 discloses the use of alkyl or alkylaryl sulfonates or phosphates and polyethoxylated alkyl phenols. U.S. Pat. No. 3,811,507 discloses the use of a water-soluble salt of a linear alkyl or alkylaryl sulfonate and a polyethoxylated alkyl sulfate.
Cationic surfactants such as quaternary ammonium salts, and derivatives of fatty amines and polyamines, have also been suggested. However, these compounds have the disadvantage of substantivity or attraction, especially towards silicate rock, and they lose their activity by adsorption.
The use of certain amphoteric surfactants which function as cationics in acid media and become anionic when incorporated in alkaline systems has been suggested. For example, U.S. Pat. No. 3,939,911 discloses a surfactant waterflooding process employing a three-component surfactant system. This surfactant system includes an alkyl or alkylaryl sulfonate such as an ammonium dodecyl benzene sulfonate, a phosphate ester sulfonate, and a sulfonated betaine such as a C.sub.12 -C.sub.24 alkylamide C.sub.1 -C.sub.5 alkane dimethylammonium propane sulfonate.
Ether sulfonates of the general formula EQU R--(OCH.sub.2 CH.sub.2).sub.n --SO.sub.3 .sup.- Na.sup.+
have been found to be effective surfactants for enhanced oil recovery. These surfactants have acceptable levels of tolerance with respect to divalent metal ions. However, these surfactants have thus far been found to be too costly for use in enhanced oil recovery.
The use of zwitterionic surfactants of the general formula ##STR2## in enhanced oil recovery is disclosed in U.S. Pat. Nos. 4,216,097; 4,370,243; 4,479,894; 4,502,540; and 4,554,974. These surfactants are prepared by reacting alkydimethylamines with 1,3-propane sultone. Propane sultone is a known carcinogen and is quite costly. Surfactants derived from propane sultone have thus far been found to be too costly for enhanced oil recovery. These surfactants also typically require the use of a cosurfactant such as alcohol to obtain desirable interfacial tension (IFT) levels. The addition of alcohol is not desirable from both a performance point of view and an economic point of view. In the reservoir, the alcohol can separate from the surfactant which leads to lower oil recovery, and the addition of alcohol adds to the overall cost.
Zwitterionic surfactants of the general formula ##STR3## are disclosed in U.S. Pat. Nos. 3,227,749 and 3,539,521 as being useful as surfactants for incorporation into soaps, detergents and other toilet articles. In the '749 patent R is defined as an alkyl chain of about 14 to about 18 carbon atoms. In the '521 patent R is defined as an alkyl radical having about 12 to about 18 carbon atoms.
The use of thickening agents to increase the viscosity of injected water, normally to a value of at least equal to that of the reservoir oil, in order to arrive at a favorable mobility ratio between the oil and water and increase the macroscopic displacement efficiency of waterflood is known. Examples of such thickeners or mobility control agents are Polysaccharide B-1459, which is available from Kelco Company under the tradename "Kelzan", and the partially hydrolyzed polyacrylamides available from the Dow Chemical Company under the tradename "Pusher" chemicals. The homopolysaccharide gum thickeners such as those available from CECA S.A. under the tradename Actigum CS 11 L have also been reported as being useful.
While many surfactants and waterflooding methods have been proposed, there is a substantial, unfilfilled need for a surfactant and waterflooding method that is useful in recovering oil from subterranean formations wherein the surfactant is exposed to high temperatures, high salinities, high pressures, and high concentrations of divalent metal ions.